1. Field
Embodiments described herein relate generally to a nonvolatile semiconductor memory device using a variable resistance element configured to store a resistance value as data.
2. Description of the Related Art
As an electrically-rewritable nonvolatile memory, a flash memories have conventionally been known in which a memory cell array is configured by NAND connection or NOR connection of memory cells each having a floating gate structure. A ferroelectric memories are also known as a memory which is nonvolatile and compatible with fast random access.
Meanwhile, as a technique for enabling greater miniaturization of memory cells, a resistance varying memory using a variable resistance element in each memory cell is proposed. Known variable resistance elements include: a phase change memory element varying a resistance by phase changes of a chalcogenide compound between a crystallized state and an amorphous state; an MRAM element using resistance changes due to tunnel magnetoresistance effect; a memory element of a polymer ferroelectric RAM (PFRAM) including a resistance element made of a conductive polymer; a ReRAM element inducing resistance changes upon electric pulse application, etc. (JP2006-344349A).
In the resistance varying memory, memory cells can be configured by a series circuit of not a transistor but a diode and a variable resistance element. Therefore, memory cells can be easily stacked to a three-dimensional structure and hence are advantageous for higher integration (JP2005-522045A).
When writing data into a ReRAM memory cell, a certain voltage is applied to the variable resistance element for a short time. In response to this, the variable resistance element changes from a high resistance state to a low resistance state. On the other hand, when erasing data from a ReRAM memory cell, a certain voltage lower than the voltage applied in the writing operation is applied for a long time to the variable resistance element which is in a low resistance state, in order to cause Joule heat by a current flowing through the variable resistance element. In response to this, the variable resistance element changes from a low resistance state to a high resistance state. The memory cells are defined as being stable when, for example, they are in a high resistance state. When the memory cells are configured to store binary data, data is written into the memory cells by a writing operation of changing the memory cells from the stable high resistance state to a low resistance state.
When erasing data from such a resistance varying memory, a certain erasing voltage is applied to a memory cell configured as a series circuit of a diode and a variable resistance element. Therefore, a voltage to be applied to the variable resistance element is determined by the resistance ratio between the diode and the variable resistance element. Hence, the instant the variable resistance element under the data erase operation changes from a low resistance state to a high resistance state, the voltage applied to the variable resistance element might rise and exceed the writing voltage to induce a so-called erroneous writing which causes the memory cell to be written again.